Re: Proposal of tunable fix for scalability of 8.4

On Mar 13, 2009, at 8:05 AM, Gregory Stark wrote:
> "Jignesh K. Shah" <J.K.Shah@Sun.COM> writes:
>
>> Scott Carey wrote:
>>> On 3/12/09 11:37 AM, "Jignesh K. Shah" <J.K.Shah@Sun.COM> wrote:
>>>
>>> In general, I suggest that it is useful to run tests with a few
>>> different
>>> types of pacing. Zero delay pacing will not have realistic number of
>>> connections, but will expose bottlenecks that are universal, and
>>> less
>>> controversial
>>
>> I think I have done that before so I can do that again by running
>> the users at
>> 0 think time which will represent a "Connection pool" which is
>> highly utilized"
>> and test how big the connection pool can be before the throughput
>> tanks.. This
>> can be useful for App Servers which sets up connections pools of
>> their own
>> talking with PostgreSQL.
>
> Keep in mind when you do this that it's not interesting to test a
> number of
> connections much larger than the number of processors you have.
> Once the
> system reaches 100% cpu usage it would be a misconfigured
> connection pooler
> that kept more than that number of connections open.


How certain are you of that? I believe that assertion would only be
true if a backend could never block on *anything*, which simply isn't
the case. Of course in most systems you'll usually be blocking on IO,
but even in a ramdisk scenario there's other things you can end up
blocking on. That means having more threads than cores isn't
unreasonable.

If you want to see this in action in an easy to repeat test, try
compiling a complex system (such as FreeBSD) with different levels of
-j handed to make (of course you'll need to wait until everything is
in cache, and I'm assuming you have enough memory so that everything
would fit in cache).
--
Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828